my son is 16, he is a long distance runner. he told me today that at times, when running fast, or walking, he seems to "lose control" on one side of the body. It doesnt always happen on the same side. There is no pain or paralysis, he explains it as a "muscle tightening", arm will "flail". This has been happening several times a week. It mostly happens at the beginning of a sprint run, lasting about 5 seconds. What could this indicate?